The need for fast and efficient optical-based technologies is increasing as Internet data traffic growth rate is overtaking voice traffic and pushing the need for fiber optical communications. Transmission of multiple optical channels over the same optical fiber in a wavelength-division multiplexing (WDM) system provides a simple way to use the unprecedented capacity (signal bandwidth) offered by fiber optics. Commonly used optical components in the system include WDM transmitters and receivers, optical filters such as diffraction gratings, thin-film filters, fiber Bragg gratings, arrayed-waveguide gratings, optical add/drop multiplexers, and tunable lasers.
Bragg gratings integrated with waveguides are especially useful in optical systems because they can be used as transmission or reflection filters, as multiplexers/de-multiplexers in WDM communication systems, or to stabilize the spectra as part of the laser's cavity in a cavity-type laser. One of the common ways of integrating Bragg grating in a waveguide is to have the desired periodicity as surface corrugation at the interface between the core and the cladding of the waveguide. The core has a different refractive index (ncore) than the cladding (ncladding), so that the optical mode traveling along the waveguide is affected by these surface corrugations as a periodic function of the waveguide's effective refractive index. Bragg gratings formed at the interface between core and cladding, however, have only a limited number of degrees of freedom and therefore a limited number of design parameters with which a designer can work to design a Bragg grating for a particular application. As a result, this type of Bragg grating has limited design flexibility.